I am negotiating the price of a 2010 M3 now, which I can supposedly get in 6 to 8 weeks. What is expected to change in March and should I wait?

Well you didn't say whether you were buying a coupe/convertible or a sedan.

If you are looking at the sedan, there is a good chance there will be no exterior changes at all. There is also a chance for some minor front end changes as well.

For the coupe/cab, exterior changes are nearly guaranteed to the rear end of the car. Pretty much just tail lights though.

I also still think there is a chance for MCB to be offered as a standard color in March, though others will strongly disagree with that. It might not happen at all or might not happen 'til MY2011 or later.

Aside from that not much else is known AFAIK. One outside possibility is front PDC for the M3 if the E92/E93 3 series get that (which I think they will).

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A gen-u-ine BMW eff-eight-zero with them tandem clutches in the transmission and that dad gum sun roof on the top-a da cawr.

I would and am waiting. I would definitely expect some new color options, and I've heard rumors of new 18 inch rims. Maybe the new 18's will be badass and you can save the cash on the 19's assuming you were going to get them? Also, I think it's very likely that the 335 will get a power boost to better compete with the Audi S4. If that happens, I can see the M3 getting something new - maybe not a power boost, but more standard options or something to add more value and keep it differentiated from the 335.

On the other hand, they might not change a damn thing and boost the price for their efforts. You never know.